Magnesium alloy and method for making the same

ABSTRACT

A magnesium alloy includes 8.7 to 11.8 wt % aluminum, 0.63 to 1.93 wt % zinc, 0.1 to 0.5 wt % manganese, 0.5 to 1.5 wt % rare earth elements, and a remainder of said magnesium alloy being composed of magnesium and unavoidable impurities.

BACKGROUND

1. Technical Field

The present disclosure relates to magnesium-based alloys having good mechanical properties, such as mechanical strength, ductility and castability.

2. Description of the Related Art

Magnesium-based alloys have been widely used as cast parts in the aerospace and automotive industries and are mainly based on the following four systems: Mg—Al system (i.e., AM20, AM50, AM60); Mg—Al—Zn system (i.e., AZ91D); Mg—Al—Si system (i.e., AS21, AS41); and Mg—Al-Rare Earth system (i.e., AE41, AE42).

Magnesium-based alloy cast parts can be produced by conventional casting methods which include die-casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting and investment casting. These materials demonstrate a number of particularly advantageous properties that have prompted an increased demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength-to-weight ratio, good castability, easy machineability and good damping characteristics. However, AS and AE alloys, while developed for higher temperature applications, offer only a small improvement in creep resistance and/or are expensive. AM and AZ alloys, are limited to low strength and poor ductility for casting.

Therefore, there is room for improvement within the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a photograph demonstrating of the microstructure of a typical magnesium alloy AZ91D.

FIG. 2 is a photograph demonstrating of the microstructure of an embodiment of a magnesium alloy of the disclosure.

DETAILED DESCRIPTION

An embodiment of a magnesium alloy contains: 8.7 to 11.8 wt % aluminum (Al), 0.63 to 1.93 wt % zinc (Zn), 0.1 to 0.5 wt % manganese (Mn), 0.5 to 1.5 wt (weight) % rare earth elements (RE), and the rest being magnesium and unavoidable impurities. RE is preferably selected from the group consisting of cerium (Ce), lanthanun (La), praseodymium (Pr), neodymium (Nd), yttrium (Y) and their combinations.

Al is an element for improving the strength of the magnesium alloy. Al tends to bind with the magnesium to form significant amounts of α-phase magnesium, and β-phase Mg₁₇Al₁₂ intermetallic compound to increase the mechanical strength. The magnesium alloys of the disclosure includes 8.7 to 11.8 wt % Al. It was found that if the magnesium alloy contains less than 8.7 wt % Al, it will not exhibit good fluidity properties and castability. If the magnesium alloy contains more than 11.8 wt % Al, the alloy tends to be brittle. The preferred ranges for Al are 8.8 to 10.8 wt %.

Zn is also an element for improving the strength. Zn tends to bind with the magnesium to form significant amounts of β-phase Mg₁₇Al₁₂ intermetallic compound to increase the mechanism strength. Zn, however, lowers the creep resistance and increases the crack sensitivity during casting. The preferred ranges for Zn are less than 1.93 wt %. The magnesium alloys which are prepared having Zn contents below the minimum amount specified above have decreased strength, castability and corrosion resistance. On the other hand, the magnesium alloys containing more than 1.93 wt %, Zn is susceptible to hot tearing and are not die castable. It was found that the Zn content should preferably be 0.63 to 1.02 wt %.

Mn forms an intermetallic compound with Al to improve the elongation of the magnesium alloy. The magnesium alloys of the disclosure have minimal amounts of unavoidable impurities such as iron, copper and nickel, to maintain a low corrosion rate. The iron content can be reduced by adding Mn. Thus, the deterioration of corrosion resistance of the magnesium alloys is restrained. It was found that, if the content of Mn is lower than 0.1 wt %, the effect is not sufficient. In addition, if the content of Mn exceeds 0.5 wt %, the yield ratio of melting deteriorates.

RE tends to transform the grain boundaries of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds to be smaller or discontinuous to form smaller grains. Thus, the RE is effective in improving the mechanism strength and ductility properties. It was found that if the content of RE is lower than 0.51 wt %, the effect for decreasing the size of grains is not sufficient. If the content of RE exceeds 1.5 wt %, the RE tends to bind with the Al to form significant amounts of Al₄RE intermetallic compound to decrease the castability. Thus a casting failure may be easily produced. Thus, the sum of RE contents is preferably higher than 0.51 wt % and lower than 1.5 wt %. In addition, MgO, H₂ impurities produced in making the magnesium alloys can be reduced by adding the RE.

A method for making a magnesium alloy of the disclosure includes following steps.

In step S1, raw materials, such as Al, Zn, Mn, RE, Mg, are melted to form a molten magnesium alloy containing: 8.7 to 11.8 wt % Al, 0.63 to 1.93 wt % Zn, 0.1 to 0.5 wt % Mn, 0.5 to 1.5 wt % RE, and the rest being magnesium and unavoidable impurities.

In step S2, the molted magnesium alloy is cast to form magnesium alloy components by casting methods, such as die-casting, thixo casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting, and investment casting and so on.

In step S3, the magnesium alloy components are re-heated to a temperature in the range of about 330 to about 420 Celsius degrees in a time in the range of about 30 to about 180 minutes. The preferred heating temperature is in the range of about 350 to about 400 Celsius degrees. The preferred heating time is in the range of 60 to 120 minutes.

In step S4, the magnesium alloy components are held for 0 to 60 minutes at the temperature in the range of about 330 to 420 Celsius degrees. The preferred holding time is in the range of 0 to 30 minutes.

In step S5, the magnesium alloy components are cooled to a room temperature.

FIG. 1 shows a photograph demonstrating of the microstructure of a typical magnesium alloy AZ91D. FIG. 2 shows a photograph demonstrating of the microstructure of an embodiment of a magnesium alloy after a heat treatment. Comparing FIG. 1 with FIG. 2 shows a great amount of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds in FIG. 2 are broken and dissolved into α-phase magnesium grains. Thus, the sum of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds is reduced, and a great amount of Al₄RE compounds are accumulated between the grain boundaries of the α-phase magnesium.

The magnesium alloys of the present disclosure were prepared in 100 kg crucible made of low carbon steel. The mixture of N₂+0.3% SF₆ is used as a protective atmosphere. The raw materials used were as follows:

Magnesium: pure magnesium, containing at least 99.8% Mg;

Aluminum: commercially pure Al (less than 0.3% impurities);

Zinc: commercially pure Zn (less than 0.005% impurities);

Manganese: in the form of Al-15% Mn master alloy;

Rare earth: Ce-rich rare earth.

Al is added into the molten magnesium during the melt heating in a temperature interval 650 to 680 Celsius degrees. Intensive stirring for 3-5 minutes is sufficient for dissolving this element in the molten magnesium. Zn is added into the molten magnesium during the melt heating in a temperature interval 650 to 680 Celsius degrees. Intensive stirring for 3-5 minutes is sufficient to dissolve this element in the molten magnesium. Al-15% Mn is added into the molten magnesium during the melt heating at a temperature in the range of 710 to 730 Celsius degrees. Intensive stirring for 20-30 minutes is sufficient for dissolving this element in the molten magnesium. RE is added into the molten magnesium during the melt heating in a temperature interval 690 to 710 Celsius degrees. Intensive stirring for 10-15 minutes is sufficient for dissolving this element in the molten magnesium.

After obtaining the required compositions, the molten magnesium alloys are held at a temperature in the range of 660-670 Celsius degrees, and then they were cast into the 7 kg rectangular components. The casting was performed with gas protection of the molten metal during solidification in the molds. Neither burning nor oxidation is observed on the surface of the all experimental components. The components of all new and comparative alloys were then re-melted and permanent-mold-cast into bars by JLM280MGIIc-type casting device, which are used for the preparation of specimens for tensile test.

The magnesium alloys of the disclosure have been tested by standard test method (ASTM-B557-02) and compared with a comparative sample (AZ91D). The chemical compositions of the magnesium alloys and AZ91D by ICP-AES are listed in Table 1. The results of mechanical property test of the magnesium alloys and AZ91D by are shown in Table 2.

TABLE 1 Chemical compositions of alloys (a remainder of said magnesium alloy being composed of Mg and unavoidable impurities) Alloys Al (wt %) Zn (wt %) Mn (wt %) RE (wt %) Comparative Example 8.7 0.71 0.20 — (AZ91D) Example 1 8.9 1.02 0.20 0.51 Example 2 8.9 1.02 0.20 0.51 Example 3 8.9 0.66 0.19 0.67 Example 4 9.2 0.63 0.18 0.79 Example 5 10.8 1.54 0.19 0.94 Example 6 10.8 1.54 0.19 0.94 Example 7 11.8 1.93 0.24 0.9 Example 8 11.8 1.93 0.24 0.9 Example 9 8.8 0.68 0.18 0.95 Example 10 8.8 0.68 0.18 0.95 Example 11 11.1 0.78 0.21 1.12 Example 12 9.1 0.71 0.24 1.23 Example 13 9.0 0.73 0.22 1.5

TABLE 2 Mechanical properties of alloys Ultimate Tensile Whether Heat Strength Elongation Alloys Treatment or not? (MPa) (%) Comparative Example No 240 5.3 (AZ91D) Example 1 No 251 7.8 Example 2 Yes 290 10.9 Example 3 No 253 8.6 Example 4 No 250 8.4 Example 5 No 249 5.4 Example 6 Yes 287 7.6 Example 7 No 258 4.5 Example 8 Yes 296 8.9 Example 9 No 255 8.3 Example 10 Yes 295 12 Example 11 No 245 5.3 Example 12 No 253 7.6 Example 13 No 246 5.5

As may be seen from the Tables, a great advantage of the alloys of the disclosure can be further seen when comparing them with AZ91D alloy with respect to ductility.

Finally, while the present disclosure has been described with reference to particular embodiments, the description is illustrative of the disclosure and is not to be construed as limiting the disclosure. Therefore, various modifications can be made to the embodiments by those of ordinary skill in the art without departing from the true spirit and scope of the disclosure as defined by the appended claims. 

1. A magnesium alloy containing: 8.7 to 11.8 wt % aluminum, 0.63 to 1.93 wt % zinc, 0.1 to 0.5 wt % manganese, 0.5 to 1.5 wt % rare earth elements, and the rest being magnesium and unavoidable impurities.
 2. Amagnesium alloy containing: 8.8 to 10.8 wt % aluminum, 0.63 to 1.02 wt % zinc, 0.1 to 0.5 wt % manganese, 0.51 to 1.23 wt % rare earth elements, and the rest being magnesium and unavoidable impurities.
 3. A method for making a magnesium alloy, comprising: (1) melting raw materials including aluminum, zinc, manganese, rare earth elements, magnesium, to form a molten magnesium alloy containing: 8.7 to 11.8 wt % Al, 0.63 to 1.93 wt % Zn, 0.1 to 0.5 wt % Mn, 0.5 to 1.5 wt % RE, and the rest being magnesium and unavoidable impurities original alloy materials; (2) casting the molten magnesium alloy to form a magnesium alloy component; (3) reheating the magnesium alloy component to a temperature in the range of about 330 to about 420 Celsius degrees by a heating time in the range of about 30 to about 180 minutes; (4) holding the magnesium alloy component for 0 to 60 minutes at the temperature in the range of about 330 to about 420 Celsius degrees; and (5) cooling the magnesium alloy component to a room temperature.
 4. The method of claim 3, wherein in the step (2), the molten magnesium alloy is cast by a casting method selected from the group consisting of die-casting, thixo casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting, and investment casting.
 5. The method of claim 3, wherein in the step (3), the temperature is in the range of about 350 to about 400 Celsius degrees.
 6. The method of claim 3, wherein in the step (3), the heating time is in the range of about 60 to about 120 minutes.
 7. The method of claim 3, wherein in the step (4), the magnesium alloy component is held for 0 to 30 minutes at the temperature in the range of about 330 to about 420 Celsius degrees. 